Currently, negatively-chargeable photoreceptors having a layered structure have been widely used as a photoreceptor installed in image forming apparatuses of an electrophotographic system such as copying machines and printers. In general negatively-chargeable photoreceptors having a layered structure, an intermediate layer and a photosensitive layer including a charge generating layer and a charge transport layer formed thereon are stacked on a conductive support. In such negatively-chargeable photoreceptors having a layered structure, the surface thereof is negatively charged and exposed to light to generate charges in the charge generating layer. Negative charges (electrons) of these charges move to the conductive support through the intermediate layer, while positive holes (holes) move to the surface of the photoreceptor through the charge transport layer to cancel negative charges on the surface, whereby forming an electrostatic latent image. Accordingly, the intermediate layer has been required to have an electron transport property (to readily transfer electrons generated in the charge generating layer by the exposure to the conductive support) and a positive hole-blocking property (to prevent injection of positive holes to the photosensitive layer from the conductive support).
Recently, image forming apparatuses of an electrophotographic system such as copying machines and printers have been required to have further improved image quality. Specific requests for improvement in image quality include improvement in uneven density in a page or between pages. In order to improve uneven density, various measures have been taken in image forming apparatuses.
Uneven density may be caused by insufficient electron transport property of the intermediate layer. Accordingly, uneven density characteristic may be solved by improving the electron transport property of the intermediate layer. For example, Patent Literature 1 discloses that the electron transport property of an intermediate layer is improved by containing metal oxide fine particles in the intermediate layer.
However, when the electron transport property of the intermediate layer is simply improved, i.e., when a particularly-sensitive charge generating material is used, injection of irregular electrons from the charge generating layer cannot be prevented, causing a problem of image defects such as spots and fogging.